For solenoids used in automotive vehicles, in many industrial applications, and in other applications involving a dusty or dirty environment, severe shortening of the operating life of the solenoid can occur if the solenoid is not adequately protected. Similar problems exist for other devices having axially reciprocal shafts; in many devices of this general kind, it is essential to protect the bearings and other elements of the operating mechanisms from the dust and dirt in the ambient environment.
One conventional protective arrangement comprises a corrugated sleeve or "boot" having one end sealed to the shaft and the other end sealed to the housing of the solenoid or other mechanism that drives the shaft through its reciprocating movement. An ordinary cylindrical sleeve or even a corrugated boot, however, is not particularly effective because the volume enclosed within the sleeve or boot varies excessively as the result of shaft movement. Thus, when the shaft moves inwardly toward the housing to which the other end of the boot is secured, the volume of the space enclosed within the boot decreases substantially. This increases the pressure on the fluid in the boot and tends to force some of that fluid out of the boot, regardless of whether the fluid is air or constitutes oil or other liquid. When the shaft subsequently moves in the opposite direction, away from the fixed end of the sleeve, the volume again expands, reducing the pressure within the boot. This tends to draw contaminated air into the boot. Thus, a protective boot of this kind, while it shields the shaft and the housing opening from direct exposure to a dirty environment, nevertheless allows the introduction of some contaminants into contact with the shaft and the housing opening, with consequent possibility of ultimate damage to the solenoid or like device.
Other protective arrangements for solenoids and similar devices having axially reciprocal shafts have included gasket seals, as shown in Stolp et al U.S. Pat. No. 1,978,916, and flexible diaphragms, as shown in Snyder U.S. Pat. No. 2,427,630 and Baier et al U.S. Pat. No. 3,142,790. Like a corrugated boot, however, when applied to a device having a shaft projecting from only one side of a housing these arrangements result in substantial changes in the volume of air or other fluid enclosed within the device. A modified diaphragm protection arrangement is shown in two United States patents to Immel, U.S. Pat. Nos. 2,858,487 and 2,853,660. The Immel arrangement uses two diaphragms instead of one, with the two diaphragms located on opposite ends of a solenoid assembly, both diaphragms being of generally cup-shaped configuration with the concave portion of one diaphragm facing the convex portion of the other. In this arrangement, when the shaft moves axially the distortion of the two diaphragms is matched and the volume of fluid, enclosed within the housing of the device, which may be air or oil, remains essentially constant. However, this construction can only be applied to a device in which both ends of the operating shaft are accessible and requires a special housing for the device.
Another arrangement for environmental protection of a reciprocating shaft in an electromagnetic device is the valve construction shown in Delaporte et al U.S. Pat. No. 3,098,635. That arrangement employs a single diaphragm which has one end sealed to the housing of the electromagnetic device and the other end sealed to the projecting shaft of the device. The diaphragm turns inside out during each stroke of the shaft and is said to maintain a constant volume of fluid in a chamber enclosing a part of the shaft, though it appears that there must be pressure changes occurring immediately before and after the diaphragm reverses its direction of curvature. Furthermore, the Delaporte arrangement requires a special housing construction and would not be readily adaptable to other types of housings, and has the additional disadvantage that repeated operation, with the diaphragm turning inside out with a 180.degree. reversal on every stroke, would be likely to lead to early fatigue and failure of the diaphragm.